1. Field of the Invention
The present invention relates to water-curable resins, and in particular, to orthopedic casting materials incorporating such curable resins.
2. The Prior Art
Many different orthopedic casting materials have been developed for use in the immobilization of broken or otherwise injured body parts. One of the first casting materials developed for this purpose was a plaster of Paris bandage.
More recently, water-curable, isocyanate-functional, polyurethane prepolymers were found to be extremely useful in formulating a resin for orthopedic casting materials, as disclosed, for example, in U.S. Pat. No. 4,411,262 (Von Bonin), U.S. Pat. No. 4,502,479 (Garwood et al.), U.S. Pat. No. 4,609,578 (Reed), U.S. Pat. No. 4,667,661 (Scholz et al.), and U.S. Pat. No. 4,774,937 (Scholz et al.). Most commonly, a knitted fiberglass fabric is used as the scrim in combination with the resin.
To initiate the cure of such water curable orthopedic casting materials, the material is contacted with water, typically by immersing a roll of the material in water. Upon immersion, the curing process begins as the isocyanate-functional groups begin polymerizing in the presence of the water. Such polymerization is often aided or controlled by the use of a catalyst, such as is disclosed, for example, in U.S. Pat. No. 4,705,840 (Buckanin). During this curing process, carbon dioxide is formed and released from the resin in the form of bubbles. Unfortunately, the evolution of carbon dioxide bubbles, if not properly controlled, can result in undesirable foaming.
Foaming is undesirable from several standpoints. First, after a roll of orthopedic casting tape has been applied, foaming can cause the end of the tape to percolate up and away from the formed cast. Second, foaming is undesirable because it can decrease layer to layer lamination and hence the strength and durability of the resultant cured orthopedic casting material. Third, foaming can adversely affect the handling characteristics of the material, and can result in undesirable dripping. During cure foaming can occur in the resin (resin phase foaming) or in the aqueous phase (aqueous phase foaming) which is created by the water used to initiate cure. Foaming occurring in the resin phase is the result of carbon dioxide being released through the resin. Resin phase foaming reduces the resin density which weakens the resin and can reduce the overall porosity of the cast through occlusion of the porous scrim. Aqueous (water) phase foaming occurs as the carbon dioxide escaping from the resin bubbles into and through the aqueous phase. Aqueous phase foaming is enhanced due to mechanical manipulation (molding) of the casting tape during application of the tape. Aqueous phase foaming often forms visible sudsing which is esthetically unpleasing and interferes with the handling properties of resin coated casting material.
Commonly assigned U.S. patent application Ser. No. 376,421 filed Jul. 7, 1989, and now abandoned, discloses the use of a stable dispersion of hydrophobic polymeric particles to reduce foaming in the aqueous phase. Such commercially available materials, however, are relatively expensive.
Several antifoaming agents, e.g. DB-100 silicone fluid (Dow Corning) as in U.S. Pat. No. 4,667,661 (Scholz et al.) and silicone Antifoam A (Dow Corning) have been used in the art to suppress foaming in the resin phase.
These latter antifoaming compounds have been effective in retarding foaming occurring within the resin but have not significantly retarded foaming occurring in the aqueous phase during curing. Although not completely understood, it is theorized that such conventional antifoaming compounds tend to remain dispersed, or otherwise entrapped within the polyurethane prepolymer resin. As a result they do not substantially enter into the aqueous phase during curing of the resin to become effective in retarding foaming occurring in that phase during cure.
Solid supported antifoaming materials are known which are composed of an antifoaming compound and a solid support material. Such solid supported antifoaming materials are disclosed for use in agricultural products, detergents, and dry beverages. Solid supported antifoaming compositions disclosed for such uses appear in U.S. Pat. Nos. 3,983,251 and 4,818,292 and in the following patent publications; Great Britain 2,220,932, European Patent publications 0,206,522 and 0,266,863. A commercially available solid supported antifoaming composition (maltodextrin solid support incorporating a polydimethylsiloxane and silica antifoaming composition) is marketed under the trade designation Dow Corning Antifoam 1920 antifoaming agent for bioprocessing.